According to the Immunization Action Coalition, there is no limit to the number of vaccines that can be administered during a single healthcare visit; vaccines should not be deferred when multiple immunizations are needed. Yet, others have called into question the practice of giving multiple vaccines at the same time due to safety concerns. Further, little is known about the psychological effects that experiencing an adverse event to one vaccine may have on a patient being receptive to other future vaccines.

Investigators conducted a cohort study to determine whether concurrent administration of the influenza and herpes zoster vaccines was associated with a reduced likelihood of receiving the influenza vaccine in the subsequent year. They undertook this study because unlike the influenza vaccine, the recombinant varicella zoster vaccine, is highly reactogenic producing, systemic reactions in about 50% of vaccine recipients. There was concern that when the vaccines are given concurrently, the perception may be that the pain and discomfort associated with the zoster vaccine is due to the flu vaccine, thereby reducing further adherence to subsequent influenza immunizations.

Data from August 1, 2018, to March 31, 2019, from a national, commercial health insurance claims database of approximately 17 million patients (including Medicare Advantage plans) were analyzed to determine patient exposure to the recombinant varicella zoster vaccine and the influenza vaccine either on the same day (concurrent administration) or 29 or 180 days before the influenza vaccine (separate administration).

While patients who received their second zoster vaccine on the same day as the influenza vaccine were included in the concurrent administration group, those who had received their zoster vaccine 1 to 28 days prior to the influenza vaccine were excluded to allow for a washout period of potential adverse effects to the zoster vaccine.

A subgroup analysis was performed on those who received concurrent administration with the first versus second dose of the zoster vaccine. Further analysis was conducted on the effect vaccination administration setting (i.e., whether the vaccine(s) was administered at a pharmacy or in an office setting) on adherence.  

Patients were required to have continuous insurance coverage from August 1, 2017, until the date of their 2018-2019 influenza vaccines. Patients were excluded if they were younger than age 50 years, had prior concurrent administration of the influenza and zoster vaccines, received multiple influenza vaccines during the 2018-2019 season, had missing sex data, lived outside of the United States, and had received the live intranasal influenza vaccine.

Adjustments were made for confounders, including sociodemographic factors, region, insurance type, month of vaccine administration, type of influenza vaccine, concurrent administration of other vaccines (e.g., pneumococcal), and comorbidities.

Of the 89,237 patients receiving both vaccines, 30.4% received the 2018-2019 influenza and zoster vaccines on the same day and 69.6% received them on separate days. The median age of the study population was 72 years, and 70% were Caucasian. Those who received the vaccines concomitantly were younger (43.4% vs. 32.9% aged <70 years, respectively) and were less likely to be enrolled in a Medicare Advantage plan (77.5% vs. 86.6%, respectively) than those who received separate administrations.

Of those who received the 2018-2019 influenza vaccine concurrently with the zoster vaccine, 87.3% received a subsequent influenza vaccine in 2019-2020, which was statistically significantly lower than the 91.3% of patients who had received the vaccines separately. The adjusted odds ratio of receiving a 2019-2020 influenza vaccine was 26% lower following concurrent versus separate administration of the influenza zoster vaccines in 2018-2019.

Those who received their concurrent vaccinations at a pharmacy were 35% less likely to receive a subsequent influenza vaccine the following year (adjusted odds of 0.65, 95% CI 0.6-0.7). Conversely, the adjusted odds ratio for subsequent influenza vaccine if the concurrent vaccines were administered in a medical office was 0.82 or only 18% lower (95% CI 0.77-0.88).

This latter finding is a bit concerning and was addressed by the authors. They commented that physicians may spend more time discussing expected vaccine adverse effects and reassuring patients and that pharmacists may need to play a greater role in educating and counseling patients on vaccine side effects.

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